1. Field of the Invention
This invention relates to a variable focal length lens system, and more particularly to a technical field of a variable focal length lens system which is used for video cameras, digital still cameras and so forth and has a zoom ratio higher than 20 times.
2. Description of the Related Art
In related art, a method is known wherein an image pickup device which uses a photoelectric conversion element such as a CCD (Charge Coupled Device) element or a CMOS (Complementary Metal Oxide Semiconductor) element as recording means for a camera is used such that the amount of light of an image of an image pickup object formed on a face of the image pickup element is converted into and recorded as an electric output.
Further, together with the progress of a microprocessing technique in recent years, enhancement of the operation speed of a central processing unit and the integration of a storage medium has been achieved. Thus, it has become possible to carry out a high speed process of a large amount of image data which has been impossible to handle in related art.
Further, enhancement of the integration and miniaturization of a light receiving element have been achieved, and such high integration has made it possible to record a higher spatial frequency and such miniaturization has achieved miniaturization of the entire camera.
However, such high integration and miniaturization of a light receiving element as described above have decreased the light receiving area of individual photoelectric conversion elements and decreases the electric output power. This has given rise to a problem that the influence of noise increases.
In order to prevent the influence of noise caused by such decrease of the electric output power as described above, several countermeasures are taken in related art. One of the countermeasures is to increase the amount of light which comes into a light receiving element by increasing the aperture ratio of the optical system. Another countermeasure is to dispose a microlens array, which includes an array of very small lenses, immediately forwardly of individual light receiving elements.
While a microlens array introduces light fluxes, which are directed to portions between adjacent ones of light receiving elements, to the light receiving elements, it provides a restriction to the position of the exit pupil of the lens system. If the position of the exit pupil of the lens system approaches the light receiving elements, that is, if the angle of a main light flux, which comes to a light receiving element, with respect to the optical axis becomes great, then an off-axis ray directed to a peripheral portion of the screen defines a great angle with respect to the optical axis. As a result, the light flux fails to come to the light receiving element, resulting in shortage in light amount.
In recent years, together with popularization of a digital camera, the needs of users have diversified. Particularly, a demand for a camera which incorporates a zoom lens having a high power variation ratio while miniaturization of the same is assured is increasing.
As one of zoom lenses of zoom types used in related art, a zoom lens of a variable focal length lens system having a four-group configuration of positive, negative, positive, and positive powers is known.
A zoom lens of the four-group configuration includes a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power, disposed in order from the object side. A zoom lens of the type described is disclosed, for example, in Japanese Patent Laid-Open No. 2006-189598 (hereinafter referred to as Patent Document 1).
Generally in a zoom lens, as the number of movable lens groups increases, the degree of freedom in selection of a zoom locus of each lens group from a wide angle end state to a telephoto end state increases. Therefore, it is known that, even if the zoom lens has a high power variation ratio, a high performance can be implemented as disclosed, for example, in Japanese Patent Laid-Open No. 2007-79194 (hereinafter referred to as Patent Document 2) or Japanese Patent Laid-Open No. 2007-292994 (hereinafter referred to as Patent Document 3). In the zoom lenses disclosed in Patent Document 2 and Patent Document 3, an additional lens group fixed in the direction of the optical axis is disposed on the image side of a known zoom lens having a four-group configuration of positive, negative, positive, and positive powers.
Also a zoom lens which achieves a high power variation ratio and a high performance is known and disclosed, for example, in Japanese Patent Laid-Open No. 2008-15251 (hereinafter referred to as Patent Document 4) although this is an interchangeable lens.
The zoom lens disclosed in Patent Document 4 includes six lens groups including a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, a fourth lens group having a negative refracting power, a fifth lens group having a negative refracting power, and a sixth lens group having a positive refracting power and disposed in this order from the object side. In the zoom lens, when the lens position state varies from a wide angle end state to a telephoto end state, the lens groups move along the direction of the optical axis such that the distance between the first and second lens groups increases, the distance between the second and third lens groups decreases, the distance between the third and fourth lens groups increases, the distance between the fourth to fifth lens groups increases, and the distance between the fifth and sixth lens groups decreases.